I traced a problem I had to an instruction in the XAMPP How-to guide, dashboard/docs/transfer-files-ftp.html. My XAMPP verion is linux-x64-5.6.14-4. I am running Ubuntu Linux 14.04. In the XAMPP "Configure FTP Access" guide, step 3 says to enter as root:
sudo usermod -G ftp susan
Obviously 'tom' in my case. The uppercase -G option causes susan to be added to the ftp group, but also REMOVED from all other groups, including the sudo and admin groups that allow susan to execute commands as root via sudo. In other words, after this command, susan can no longer run sudo! The only recovery is to add susan back to those groups, which is now impossible because adduser can be run only by root. Arghhhh!
I recovered by booting in recovery mode (hold down shift) and dropping into root mode, where i was able to repair the sudoers file in /etc/ and run adduser tom (in my case) back to the sudo group. I was then able to reboot and run sudo again, which is required to start lampp.
This advice in the XAMPP how-to guide cost me all morning -- hey I'm not complaining, after I all I learned some things -- but these instructions should be revised. I don't know what the proper command would be to usermod susan into the ftp group, but some way should be noted here NOT to remove users from other groups, especially sudo on Ubuntu systems.
If anybody else has this trouble, http://psychocats.net/ubuntu/fixsudo is an excellent article on how to recover from this trouble. Hope this info is useful. Comments?